Degradation of Arachidonoyl‐Labeled Phosphatidylinositols by Brain Synaptosomes

Abstract

Incubation of synaptosomes together with 1-acyl-2-[14C]arachidonoyl-sn-glycerophosphoinositols (GPI) and sodium deoxycholate yielded diacylglycerols and free arachidonic acid. Diacylglycerol formation is attributed to hydrolysis by the diacyl-GPI-specific phospholipase C (EC 3.1.4.10), and this reaction requires sodium deoxycholate for optimal activity. The free arachidonic acid formed is attributed to hydrolysis of diacyl-GPI by phospholipase A (EC 3.1.1.5). Free fatty acid release was observed during incubation, even in the absence of bile salts, but this process was preferentially stimulated by sodium taurocholate. The release of fatty acids was not specific for diacyl-GPI, as similar release was obtained during incubation with other phosphoglycerides. In the presence of deoxycholate (2 mg/ml), the release of diacylglycerols was maximal at a diacyl-GPI concentration around 1.0 mM. However, the free fatty acid release was linear with respect to the substrate at least up to 1.4 mM. The rate of diacylglycerol release from diacyl-GPI was more rapid in the initial 30 min, whereas the free fatty acid release was linear with time up to 2 h. Under this incubation condition, calcium was found to stimulate both types of hydrolytic action, although the concentration needed to achieve this stimulation was rather high. This type of labeled precursor is potentially useful for studies of the different modes of diacyl-GPI degradation by enzymes in brain subcellular membranes.

Stimulation of Phospholipase A2 and Secretion of Catecholamines from Brain Synaptosomes by Potassium and A23187

“…The high incorporation of arachidonic acid into phosphatidylinositol of mouse brain has been reported by Sun et al (1979). Der and Sun (1981) also reported the presence of a phospholipase C specific for phosphatidylinositol in brain synaptosomes.…”

Essential and non‐essential phosphatidylinositol residues in acetylcholinesterase and arylacylamidase of sheep basal ganglia

“…Phosphatidylinositol (PI)-specific phospholipase C has found widespread use in the solubilization of membrane-bound enzymes [4][5][6]. Moreover, sodium deoxycholate (DOC) activated the endogenous PI-specific phospholipase C in the brain and platelets [7][8][9]. Here we show that PIspecific phospholipase C of S. aureus or DOC-activated endogenous phospholipase C solubilizes AChE and AAA of sheep brain basal ganglia and that beyond a critical [DOC], a loss in activity of these enzymes occurs which can be restored by externally added PI.…”

The Catabolism of Phosphatidylinositol by an EDTA‐Insensitive Phospholipase A1and Calcium‐Dependent Phosphatidylinositol Phosphodiesterase in Rat Brain

“…Phosphatidylinositol can also be metabolised by lysosomal enzymes, which consist of both a Ca2 +-independent phosphodiesterase and phospholipase-A-type enzymes which bring about a complete deacylation of the phospholipid to glycerophosphoinositol [12, 131. Recently, Shum et al [I41 have reported that the deacylation of phosphatidylinositol by phospholipase A, and A, may occur in a rat brain microsomal fraction as measured by the release of the fatty acids from phosphatidylinositol in the presence of 5 mM CaCI,. It has also been reported that a phospholipase A (most likely A,) active against phosphatidyl- inositol exists in mouse brain synaptosomes [15]. Finally, when phosphatidylinositol was incubated with a soluble extract from rat brain, a small amount of free fatty acid was released although the major products were phosphoinositol and diacylglycerol [3].…”